Some Current Topics of Interest in the Physiology of Trigeminal Pain: A Review

Sensory ability in the narwhal tooth organ system

The erupted tusk of the narwhal exhibits sensory ability. The hypothesized sensory pathway begins with ocean water entering through cementum channels to a network of patent dentinal tubules extending from the dentinocementum junction to the inner pulpal wall. Circumpulpal sensory structures then signal pulpal nerves terminating near the base of the tusk. The maxillary division of the fifth cranial nerve then transmits this sensory information to the brain. This sensory pathway was first described in published results of patent dentinal tubules, and evidence from dissection of tusk nerve connection via the maxillary division of the fifth cranial nerve to the brain. New evidence presented here indicates that the patent dentinal tubules communicate with open channels through a porous cementum from the ocean environment. The ability of pulpal tissue to react to external stimuli is supported by immunohistochemical detection of neuronal markers in the pulp and gene expression of pulpal sensory nerve tissue. Final confirmation of sensory ability is demonstrated by significant changes in heart rate when alternating solutions of high-salt and fresh water are exposed to the external tusk surface. Additional supporting information for function includes new observations of dentinal tubule networks evident in unerupted tusks, female erupted tusks, and vestigial teeth. New findings of sexual foraging divergence documented by stable isotope and fatty acid results add to the discussion of the functional significance of the narwhal tusk. The combined evidence suggests multiple tusk functions may have driven the tooth organ system's evolutionary development and persistence.

Observations on facial nociception in a monkey after destruction of the rostral part of the trigeminal sensory nuclear complex

Facial sensibility was assessed in a cynomolgus monkey in whom the trigeminal main sensory nucleus and the rostral part of the trigeminal nucleus oralis on one side had been destroyed. The animal responded equally to noxious mechanical stimuli applied to the two sides of the face; no deficiency in nociception could be detected. This finding suggests that the synaptic connections made by nociceptor afferents from the face in rostral parts of the trigeminal sensory nuclear complex are not essential for facial nociception. These observations are also consistent with the opinion that, from a functional point of view, facial nociceptor afferents have their principal synaptic connections in the nucleus caudalis of the trigeminal spinal complex.